Factors that influence the performance of a golf club include the primary moment and secondary moment (moment of inertia) of the golf club. The primary and secondary moments are respectively expressed as m×L and m×L2 where m is the mass of the golf club and L is the length from the grip side end of the golf club to the barycentric position of the golf club.
Japanese Patent Laid-Open Nos. 8-173577 and 2005-198816 disclose a golf club improved based on its primary moment. These references point out that when the primary moment is decreased, the golfer can easily increase the head speed and can expect that the traveling distance of a ball will increase. On the other hand, the golf club becomes lighter and the swing orbit of the golf club becomes unstable thus degrading the direction of the ball.
Japanese Patent Laid-Open No. 2001-46563 discloses a golf club improved by considering the secondary moment. Generally, it is known that the secondary moment of a golf club influences the feel at impact. When the secondary moment is large, the feel at impact is enhanced, and the golfer can easily swing the golf club fully. Also, the direction of the ball improves.
When the secondary moment is increased, however, the primary moment also increases accordingly. Consequently, the head speed decreases and reduces the traveling distance of the ball. Conventional golf clubs have been improved based on only either the primary or secondary moment and is difficult to increase the traveling distance of the ball and at the same time enhance the feel at impact.